Molded circuit breaker operating mechanisms and trip units are evolving on a continuing basis in an attempt to completely automate the circuit breaker assembly process. U.S. Pat. No. 4,622,530 entitled "Circuit Breaker Assembly For High Speed Manufacture" describes one such molded case circuit breaker. The "calibration yield", that is, the number of circuit breakers that successfully pass an in-line circuit breaker calibration test assembled in accordance with the teachings contained within this Patent is in excess of ninety-five percent. The circuit breaker high calibration yield latch assembly, that interfaces between the circuit breaker trip unit and the circuit breaker operating mechanism, is designed to minimize the differences in the latching force produced by the circuit breaker latch mechanism.
U.S. Pat. No. 4,679,016 entitled "Interchangeable Mechanism For Molded Case Circuit Breaker" describes the use of the aforementioned high calibration yield latch assembly with circuit breakers having an operating mechanism that is designed for high speed manufacture. The precision alignment of the circuit breaker components, by robotic assembly, decreases the tolerances that would otherwise exist between the various components, when hand assembled. The precise alignment of the circuit breaker components along with the improved latch arrangement further decreases the number of circuit breakers rejected during the in-line calibration process. Both Applications are incorporated for purposes of reference and should be reviewed for the teachings contained therein.
U.S. patent application Ser. No. 941,974 filed Dec. 15, 1986, entitled "Molded Case Circuit Breaker Contact Arrangement" describes the elimination of the circuit breaker flexible braid conductor that is otherwise required for connecting the bimetal trip units with the circuit breaker movable contact arm. The drag resulting on the movable bimetal when such braided conductors are attached, can interfere with the circuit breaker's thermal calibration. This Application is incorporated herein for purposes of reference.
U.S. Pat. No. 4,630,019 entitled "Molded Case Circuit Breaker With Calibration Adjusting Means For a BiMetal" describes an intermediate control lever coupled to a relatively stationary portion of a bimetal to enable adjustment thereof without applying undue stress to the movable portion of the bimetal.
It has since been determined that the variation between bimetal trip units used in circuit breakers relates to the difficulty in welding the bimetal strip to the heater at exactly the same spot. This changes the fulcrum point at which the bimetal exerts its motive force on the circuit breaker trip bar and hence the amount of force applied thereto. The welding of the bimetal element to the heater, per se, thermally overstresses the bimetal causing a change in the bimetal's characteristics. This introduces inaccuracies into the active area of the bimetal available for responding to predetermined overcurrent values during the circuit breaker calibration process.
One purpose of the instant invention is accordingly to provide a bimetal trip unit arrangement whereby the active region of the bimetal is unaffected by thermal over-stress conditions occurring during the welding process. A further purpose of this invention is to set the exact location of the fulcrum point along the bimetal.